Reader monad inside RxJs mergeMap

Question

I am using rxjs and want to use Reader monad from fp-ts package as a Dependency Injection solution.

Here is my code:

import { of } from 'rxjs';
import { pipe } from 'fp-ts/function';
import { mergeMap } from 'rxjs/operators';
import * as R from 'fp-ts-rxjs/ReaderObservable';

type Dependency = { dep1: string }

const fn1 = (input: string): R.ReaderObservable<Dependency, string> => (dep: Dependency) =>
  of(`${input} | ${dep.dep1}`);

const fn2 = () => pipe(
  of('initial').pipe(
    mergeMap(x => pipe(
      fn1(`| inside merge map ${x}`),
    )),
  ),
);

fn2()({dep1: "something"}).subscribe(
  data => console.log(data),
);

fn1 function has a dependency that is injected using Reader monad

The problem is when I use this function inside a mergeMap the return value is a ReaderObservable and not an Observable and causes errors.

How can I use ReaderObservable inside a mergeMap?

Answer 1

It doesn't work because mergeMap's callback must return either an Observable or an ObservableInput:

type ObservableInput<T> = SubscribableOrPromise<T> | ArrayLike<T> | Iterable<T>;

But a ReaderObservable is a function that returns an Observable.

Anything that expects Observable | ObservableInput won't be able to use it without actually invoking it with the dependency.

---

So how can you use it? Well, at what point is your dependency actually going to be injected? Your code example doesn't show the actual point of injection. At a minimum, the dependency must be in scope within the mergeMap callback so you can convert the ReaderObservable to an actual Observable.